Chlorine for the Pool

Introduction

Chlorine is a popular and versatile chemical; it is used in some processes of water purification, in disinfectants, and in bleach. Most importantly, and indeed what we're looking at in this article, is it's use prolific use in public and private swimming pools

In this article

• What is chlorine? - the basic stuff
• Ten essential chlorine facts
• Chlorine – the more scientific stuff
• How is chlorine produced?
• Why is it used in the pool?
• How does chlorine clean the pool?
• Not just chlorine for pools
• Other uses for chlorine
• Related Articles
  

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What is chlorine? - the basic stuff

Chlorine is smelly, poisonous chemical used extensively in bleaches and disinfectants. It is made from sodium chloride (common salt, to you and me), and plain old water; it takes a little more than simply putting the two ingredients in the container to produce some, but that is all it is. We're interested in the bits of it that make it good for use in our pools.

Ten essential chlorine facts

Chlorine is…

1. a gas at room temperature and pressure;
2. yellowish green;
3. good for combining directly with almost all elements (meaning it is very volatile, and that it mixes well);
4. smelly: breathing around 4 parts per million (ppm) can be detected by humans;
5. poisonous: around 1000ppm will be quickly fatal;
6. named after the Greek for ‘green': khloros
7. known to have been discovered in 1774 by Swedish chemist Carl Wilhelm Scheele;
8. actually naturally occurring in humans, and is found in teeth, blood and skin;
9. even present in our white blood cells, and is used to fight off infections.
10. Directly or indirectly responsible for more than 2,000,000 jobs in European industry

Chlorine – the more scientific stuff

Chlorine has a chemical symbol of 'Cl', and an atomic number of 17 (meaning it has 17 protons and electrons, unless it's a special isotope, which we'll discuss later), it has 18 neutrons, and its atomic mass is (roughly) 35.4527 amu (atomic mass units). The melting point is –100.98 degrees centigrade (-149.76 degrees Farenheit or 172.17 Kelvin), and its boiling point is –34.60 degrees C (-30.28 F or 236.55K) – this means that at normal room temperature it manifests itself in gaseous form. Unless you live somewhere really cold (possible, maybe), or somewhere with a really high pressure (less likely).

Chlorine has a orthorhombic crystal structure, and is classified as a Halogen. Check out the periodic table and you'll see much of this for yourself.

Cl-35 (18 neutrons) is the standard formation of Chlorine, and this is stable (in a radioactive sense), as is Cl-37 (19 neutrons). Cl-36 does decay, but the rate is, uhm, slow (it has a half-life of 301000 years, so don't go worrying about it). Cl-38 is the bad boy with a half-life of only 37.3 minutes. Fortunately you probably won't be getting much of this in your pool, though (unless you have a particle accelerator set up there), since it decays so quickly that we're very unlikely to ever encounter any.

Chlorine gas is diatomic, meaning, like girls going to the toilet, that it only travels in pairs. The chemical formula for diatomic chlorine gas is Cl2.

How is chlorine produced?

Not so much 'produced', but extracted. But if I'd called this section 'how is chlorine extracted' I would have given some of the secret away. You have to maintain an air of mystery, you know.

Chlorine comes from salt, as you may have noticed from earlier reading, and the EuroChlor website gives us the rather amazing statistic that "About 50 quadrillion tonnes of dissolved sodium chloride (common salt) are found in the world's oceans and seas". More amazingly, "less than a third of salt production stems from seawater.", with the majority coming from a number of rock salt mines placed hundreds of meters below the Earth's surface. Wow. The chloride ion component that help make up chlorine bleach make up about 1.9% by mass of seawater.

There is a LOT of rock salt under the Earth's surface. Huge machines tunnel their way into the rock, lots of explosives are placed in the hole that has been made, and then all the workers leave – quickly – before ceiling is literally brought down with a large explosion. The resulting rock salt is then transported by conveyor belt to the surface.

Chlorine gas is produced from this sodium – usually when the salt is dissolved by electrolysis - as a result of this equation:

As you can see we get two parts sodium chloride, 2 parts water, and get 1 part chlorine gas, one part hydrogen gas, and two parts sodium hydroxide. Fun!

Why is it used in the pool?

Placing chlorine into a swimming pool does a number of really useful things, but most importantly it kills the bacteria that live, work, feed and breed there. Without a cocktail of chemicals, your pool will likely turn a very (un)pleasant shade of slime-green. While it's pretty to look at, and while you might be able to wow the conservationists in your area with the wide variety of endangered wildlife that you've accumulated, it's simply not much good for swimming and playing in. And let's be honest: you didn't buy a pool to breed algae in, did you?

If you did, then naturally you can stop reading here.

Still with me? Good. So you want a clean pool. And you want one that is safe to swim, bathe and play in. Very good. You'll be needing some chlorine then. Chlorine is actually commonly used in water supplies (on a very small scale), along with a number of other tasty chemicals, and if nothing else that shows that a properly treated pool is safe enough to be in. It doesn't mean that we're encouraging you to go and fill the kettle up in the pool, though, so don't blame us when you drink the chlorinate goodness and fall foul of some nasty disease or other.

How does chlorine clean the pool?

When chlorine (Cl₂) is added to water (H₂0), a reaction occurs. It looks something like this:

Cl₂ + H₂O = HOCl + HCl

You may or may not recognise that HOCl is hypochlorous acid (HCl is the slightly more commonly encountered hydrochloric acid). Hypochlorous acid is really quite a nasty chemical, and it is this that does all the hard sanitisation work; it kills microorganisms by breaking through cell walls and destroying the inner enzymes, structures and processes. It can do this to skin, and that's why too much chlorine is at best irritating, and at worst very dangerous.

Not just chlorine for pools

Pure chlorine is used mostly in commercial pools, but variations on it (its chemical brothers and sisters, if you like) are widely used in non-commercial pools. Sodium hypochlorite (which is otherwise known as household bleach) is often used as a substitute; sodium hypochlorite is a compound of chlorine, combined with some sodium and oxygen. A cheaper version of this is calcium hypochlorite. We really do strongly recommend that you buy specialist pool chemicals, however, and that you don't go putting bleach in the pool.

Other uses for chlorine

Other uses of chlorine completely unrelated to your swimming pool include poison gas (World Was I say this natty invention), chlorine bombs (picked up by those ingenious insurgents in Iraq, as recently as 2007), and chloroform. Happy days!

Related Articles

• Swimming Pool Chlorine
• Swimming Pool Chemicals
• How Much Chlorine Should I Put In My Pool?